1. Field of the Invention
The present invention relates to a novel piezoelectric resonator and to an electronic component containing the novel piezoelectric resonator. More particularly, the present invention relates to a piezoelectric resonator which maximizes the efficient use of mechanical vibration of a piezoelectric member and also relates to an electronic component such as a ladder type filter, an oscillator or a discriminator containing the novel piezoelectric resonator.
2. Description of the Related Art
FIG. 19 is a perspective view of a laminated piezoelectric resonator 1 which the applicant of the present invention proposed in Japanese Patent Application No. 8-110475 which has not been published or laid open. FIG. 20 is a diagram showing the disposition of external electrodes on the piezoelectric resonator. The laminated piezoelectric resonator 1 preferably includes a laminated body 2 which preferably has a substantially rectangular block shape and which is formed by laminating a plurality of piezoelectric layers 3 and internal electrodes 4 interposed between the piezoelectric layers 3 so as to form an integral unit. Each piezoelectric layer 3 is preferably formed of a piezoelectric ceramic. The internal electrodes 4 are positioned a predetermined distance apart from each other and have major surfaces arranged to be substantially perpendicular to the longitudinal direction of the laminated body 2.
The piezoelectric layers 3 are polarized along a longitudinal direction of the laminated body 2 so that the adjacent pair of the piezoelectric layers 3 on the opposite sides of each internal electrode 4 are polarized in opposite directions, as indicated by the arrows in FIG. 19, thereby forming a vibrating section 5 or active section.
Strips of insulating film 6 and strips of insulating film 7 are respectively disposed on two opposite side surfaces of the laminated body 2. Edges of the internal electrodes 4 exposed at a first of the opposite side surfaces of the laminated body 2 are alternately covered and left uncovered with insulating film 6. Edges of the internal electrodes 4 exposed at a second of the side surfaces of the laminated body 2, those opposite from the exposed edges not covered with insulating film 6 in the first side surface are covered with insulating film 7.
Further, on the side surfaces of the laminated body 2 on which insulating films 6 and 7 are disposed, external electrodes 8 and 9 for functioning as input/output electrodes are also disposed. The external electrode 8 is connected to the internal electrodes 4 not covered with insulating film 6 while the external electrode 9 is connected to the internal electrodes 4 not covered with insulating film 7. That is, each adjacent pair of the internal electrodes 4 are respectively connected to the external electrodes 8 and 9.
When an alternating current signal having a voltage, the direction of which changes with time, is applied to the laminated piezoelectric resonator 1, the laminated body 2 vibrates in the longitudinal direction. That is, when an electric field is applied to each piezoelectric layer 3 in the same direction as the polarization, each piezoelectric layer 3 expands in the longitudinal direction of the laminated body 2 and the entire laminated body 2 expands along the longitudinal direction. When an electric field is applied to each piezoelectric layer 3 in the direction opposite to the direction of the polarization, each piezoelectric layer 3 contracts in the longitudinal direction of the laminated body 2 and the entire laminated body 2 also contracts along the longitudinal direction. These movements are alternately repeated.
In the piezoelectric resonator 1 constructed as described above, all the directions of expansion or contraction of the piezoelectric layers 3 caused when electric fields are applied thereto are the same. In the piezoelectric resonator 1, therefore, an input signal can be efficiently converted into mechanical vibration and the electromechanical coupling coefficient can be increased, so that .DELTA.F which is the difference between the resonant frequency F and the antiresonant frequency Fa is comparatively large. Therefore, the piezoelectric resonator 1 having such characteristics is suitable for use as, for example, a wide band filter.
FIG. 21 shows an example of a ladder type filter constructed by combining a plurality of laminated piezoelectric resonators 1. The ladder type filter 100 shown in FIG. 21 includes a substrate 102 preferably formed of alumina. Thick film electrodes 104 are provided on a surface of the substrate 102. The thick film electrodes 104 are formed of patterns on the substrate 102 so as to establish connections such as those indicated by the broken lines in FIG. 21. The laminated piezoelectric resonators 1 each having the above-described structure are mounted on the substrate 102 preferably with an electroconductive paste 106 interposed therebetween. One external electrode 9 of one of the laminated piezoelectric resonators 1 is connected to one of the pattern electrodes 102 on the substrate 102 preferably by electroconductive paste 106 while the other external electrode 8 is electrically connected to the external electrode 8 of another of the laminated piezoelectric resonators 1 or to one of the thick film electrodes 104 on the substrate 102. A metallic cap 112 for protecting the laminated piezoelectric resonators 1 is bonded to the substrate 102 by an adhesive 110.
In the above-described laminated piezoelectric resonator 1, a pair of external electrodes 8 and 9 are disposed on opposite side surfaces or on a common side surface and are used for connection to an external circuit or a different kind of electronic circuit.
In the above-described laminated piezoelectric resonator 1, however, since the external electrode 8 is disposed, for example, on only one of four side surfaces of the cuboid laminated body 2 as shown in FIG. 20, connection to the external electrode 8 is impossible on the other three side surfaces. The same can also be said with respect to the external electrode 9. Thus, the above-described laminated piezoelectric resonator 1 has a directionality when mounted on a substrate or the like and there is a need for a process step of arranging laminated piezoelectric resonators 1 so that each piezoelectric resonator 1 faces in a predetermined direction in the process of manufacturing an electronic component.
Due to such a mounting directionality, considerable restrictions are imposed on the design and manufacture of a ladder type filter which is formed by combining a plurality of laminated piezoelectric resonators 1, so that it is difficult to reduce the size and manufacturing cost of the filter. For example, in the case of the ladder type filter 100 shown in FIG. 21, there is a directional restriction on mounting laminated piezoelectric resonators 1 such that the laminated piezoelectric resonators 1 must be arranged in a direction parallel to the surface of the substrate 102, thereby requiring that the mounting area of the substrate 102 be disadvantageously large. Moreover, there is also a problem with complicated wiring using wires 104 required for connecting external electrodes 8 or 9.